Scientists and clinicians face a number of challenges in their attempts to develop active agents into forms suited for delivery to a patient. Active agents that are polypeptides, for example, are often delivered via injection rather than the oral route. In this way, the polypeptide is introduced into the systemic circulation without exposure to the proteolytic environment of the stomach. Injection of polypeptides, however, has several drawbacks. For example, many polypeptides have a relatively short half-life, thereby necessitating repeated injections, which are often inconvenient and painful. Moreover, some polypeptides may elicit one or more immune responses with the consequence that the patient's immune system may be activated to degrade the polypeptide. Thus, delivery of active agents such as polypeptides is often problematic even when these agents are administered by injection.
Some success has been achieved in addressing the problems of delivering active agents via injection. For example, conjugating the active agent to a water-soluble polymer has resulted in polymer-active agent conjugates having reduced immunogenicity and antigenicity. In addition, these polymer-active agent conjugates often have greatly increased half-lives compared to their unconjugated counterparts as a result of decreased clearance through the kidney and/or decreased enzymatic degradation in systemic circulation. As a result of having greater half-life, the polymer-active agent conjugate requires less frequent dosing, which in turn reduces the overall number of painful injections and inconvenient visits to a health care professional. Moreover, active agents that are only marginally soluble often demonstrate a significant increase in water solubility when conjugated to a water-soluble polymer.
Due to its documented safety as well as its approval by the FDA for both topical and internal use, polyethylene glycol has been conjugated to active agents. When an active agent is conjugated to a polymer of polyethylene glycol or “PEG,” the conjugated active agent is conventionally referred to as having been “PEGylated.” The commercial success of PEGylated active agents such as PEGASYS® PEGylated interferon alpha-2a (Hoffmann-La Roche, Nutley, N.J.), PEG-INTRON® PEGylated interferon alpha-2b (Schering Corp., Kennilworth, N.J.), and NEULASTA™ PEG-filgrastim (Amgen Inc., Thousand Oaks, Calif.) demonstrates that administration of a conjugated form of an active agent can have significant advantages over the unconjugated counterpart. Small molecules such as distearoylphosphatidylethanolamine (Zalipsky (1993) Bioconjug. Chem. 4(4):296-299) and fluorouracil (Ouchi et al. (1992) Drug Des. Discov.9(1):93-105) conjugated to poly(ethylene glycol) have also been prepared. Harris et al. have provided a review of the effects of PEGylation on pharmaceuticals. Harris et al. (2003) Nat. Rev. Drug Discov. 2(3):214-221.
Despite these successes, conjugation of a water-soluble polymer to an active agent remains challenging. One such challenge is the deactivation of the active agent upon attachment to a relatively long polyethylene glycol molecule. Although a relatively long polyethylene glycol molecule would provide the corresponding active agent-polymer conjugate with greater water solubility, conjugates bearing such long polyethylene glycol moieties have been known to be substantially inactive in vivo. It has been hypothesized that these conjugates are inactive due to the length of the relatively polyethylene glycol chain, which effectively “wraps” itself around the entire active agent, thereby blocking access to potential ligands required for activity.
The problem associated with inactive conjugates bearing a relatively large polyethylene glycol moiety has been solved, in part, by using “branched” forms of a polymer. An example of such a “branched” polymer is described in U.S. Pat. No. 5,932,462 to Harris et al. As described therein, “mPEG2-N-hydroxysuccinimide” can be attached to an accessible amino group (e.g., an amino group that is not physically blocked due to conformational structure) on a biologically active protein. This branched polymer (having a molecular weight of about 40,000 Daltons) is available from Nektar Therapeutics (Huntsville, Ala.) and has the following structure:
wherein mPEG20K represents a methoxy-end capped polyethylene glycol derivative having a molecular weight of about 20,000 Daltons.
Coupling of this branched polymer to interferon alpha-2a results in a conjugate containing an amide bond linking interferon alpha-2a to the polymer. Schematically, the conjugate can be represented as follows:
This conjugate, available commercially as PEGASYS® brand of PEGylated-interferon alpha-2a (Hoffmann-La Roche, Nutley, N.J.), is indicated for the treatment of hepatitis C in adults.
Although utilizing a branched polymer may solve some of the problems associated with relatively large linear polymers, other challenges to preparing useful conjugates persist. For example, the in vivo rate of degradation of the conjugate is often unacceptably either too long or too short. Specifically, the in vivo rate of degradation is generally (although not necessarily) partially governed by the rate of hydrolysis occurring at some point in the series of atoms that link the active agent to the polymer. Thus, a relatively quick hydrolytic rate can result in a unacceptable conjugate having too short of an in vivo half-life while relatively slow hydrolysis can result in a unacceptable conjugate having too long of an in vivo half-life. Consequently, polymers having a unique series of atoms (both in the polymer itself as well as in the corresponding conjugate) can result in unique rates of hydrolysis, which in turn influence the in vivo rate of degradation of the conjugate.
Hydrolysis of conjugates of certain active agents and mPEG2-N-hydroxysuccinimide occurs in the chain of atoms that connects one mPEG “branch” to the other, given that one of the metabolites has a molecular weight of about twenty thousand Daltons. One likely location in the chain of atoms for such a cleavage is within the
moiety located immediately adjacent to one of the MPEG portions in the polymer. The
moiety represents the most likely location for cleavage because the only other atoms in the chain connecting one mPEG branch to the other are a series of carbon atoms comprised of methylene groups, which are relatively more stable to in vivo degradation than the
moiety. Upon cleavage, the separated form of the polymer is mPEG-OH. Thus, based at least in part on the favorability of forming mPEG-OH, a unique rate of hydrolysis results.
It would be desirable, however, to be able to provide polymers such that their hydrolysis rates could be “customized.” For example, with respect to the typical weekly administration of PEGylated interferon alpha-2a, a slower rate of hydrolysis might provide for even longer periods between administrations. In addition, conjugates having too long of an in vivo half life could be improved by increasing the conjugates' susceptibility to hydrolysis.
An expanded palette of polymers having unique hydrolysis rates would enable researchers and scientists to provide active agent-polymer conjugates “customized” to provide (among other things) the desired increase in water solubility and/or rate of degradation in vivo. Moreover, polymers having unique hydrolysis rates could be used not only for branched polymers, but other forms (e.g., linear or multiarm) as well. Thus, there remains a need for polymers that provide (among other things) a unique series of atoms to provide “customized” degradation rates. To the best of applicants' knowledge, the presently described polymers, conjugates, preparations, and methods are novel and completely unsuggested by the art.